FORMA TION OF ME TEORFFES 3 7 I 



while that of a loo-mile-cube is as i to 636. It will be seen, 

 therefore, that in a comparatively small body the cohesive 

 resistance to disruption bears a very small relation to the gra\'ity 

 of the mass, and that for large bodies it is negligible. For such 

 bodies, the Roche limit ma}' be taken as appreciably the limit 

 of the sphere of disruption. 



These numerical considerations, however, show that frag- 

 mentation by differential gravity acting alone will not become 

 minute in any such case as that of a satellite or asteroid making 

 a near approach to one of the planets. 



But there are additional considerations that influence the 

 practical result. The outer portion of the earth, and doubtless 

 that of the satellites, asteroids, and cold planets generally, is 

 deeply traversed by fissures — oblique and horizontal as well as 

 vertical — which render it little more than a pavement of dissev- 

 ered blocks which could be lifted away with little resistance 

 beyond that of gravity. The relief of pressure upon the less 

 fissured portion below, which would follow upon the removal of 

 the overlying fissured portion, and the sudden exposure of this 

 under portion to a lower temperature resultant from this removal, 

 would develop new stresses ; and these would doubtless give 

 rise to additional fissuring and further easy removal, and thus 

 the process would be extended. It is not improbable that the 

 sudden rending open of a sphere that is hot within and the 

 consequent exposure of the highly heated rocks in the interior 

 to much lower temperatures would result in sufificiently great 

 differential contraction to minutely disrupt the fragments irre- 

 spective of differential gravitation. The central portions of a 

 body sufficiently hot to melt at surface pressures would doubtless 

 pass immediately into the liquid condition on the removal of the 

 pressure of the overlying rock, and this passage might, not 

 unlikely, take on eruptive violence by reason of the included 

 and highly compressed gases — or substances in a potentially 

 gaseous state — in which case an extremely minute division 

 would ensue. In the case of the earth, there is sfood reason to 

 believe that if its interior gravitative stresses were suddenly 



